(a) Field of the Invention
The present invention relates to a method for measuring thickness or a surface profile, and more particularly to a method for measuring thickness or a surface profile, which employs a white light scanning interferometry to correctly measure the thickness of a permeable thin film layer and the surface profile of the thin film layer.
(b) Description of the Related Art
Since control of the thickness of a thin film layer is a great deal of weight among many factors determining quality in a semiconductor process and a flat panel display (FPD) process, it is necessary to directly monitor the thickness of the thin film layer during the process. The ‘thin film layer’ is a layer formed on a base layer, i.e., a substrate and having a very thin thickness, which generally has a thickness of several tens Å˜several μm. To apply these thin film layers to a certain use, it is needed to know the thickness, composition, roughness, and other physical and optical properties of the thin film layer. In particular, it has recently been a general trend for forming multiple ultra thin films on the substrate to increase integration of a semiconductor device. To develop such a highly-integrated semiconductor device, a property of matter such as the thickness of the thin film layer has to be precisely controlled as a factor of largely affecting the properties.
There are many methods of measuring the thickness of the thin film layer, which are employed in the semiconductor process and other applicable processes. Among many methods, a mechanical method using a stylus, an optical method, or the like are most general. In the case of the optical method, a white light interferometer may be used to determine the thickness of the thin film layer.
FIG. 1 shows an example of a conventional method for measuring the thickness.
Referring to FIG. 1, permeable thin film layers 20a and 20b, the thickness of which will be measured, are formed on a base layer 10, and an air layer 30 is provided above the thin film layers 20a and 20b. First surfaces 21a and 21b indicate interfaces between the air layer 30 and the thin film layers 20a and 20b, and second surfaces 11a and 11b indicate interfaces between the thin film layers 20a and 20b and the base layer 10. The left thin film layer 20a is thicker than the right thin film layer 20b. 
If the thin film layer 20a having a relatively thick thickness is illuminated with white light by the white light interferometer, a first waveform 41 is got from the first surface 21a and a second waveform 42 is got from the second surface 11a. The thin film layer 20a is so thick that the first waveform 41 and the second waveform 42 are not interfered but separated. Thus, it is possible to obtain the thickness of the thin film layer 20a by selecting the maximum peaks of the two waveforms 41 and 42.
However, in the case of the thin film layer 20b having a relatively thin thickness, it is impossible to obtain the thickness of the thin film layer 20b through the above method. That is, if the thin film layer 20b having a relatively thin thickness is illuminated with white light, there is interference between a first waveform 43 got from a first surface 21b and a second waveform 44 got from a second surface 11b. Thus, the interference makes it unclear whether a peak shown in the first and second waveforms is a peak based on real constructive interference or a peak based on interference between the first waveform 43 and the second waveform 44. Therefore, there is a problem that it is impossible to use a method of obtaining the thickness of the thin film layer 20b by selecting the maximum peaks of the two waveforms 43 and 44.